Polyamino acids such as polyaspartic acid (PAA) are useful in fertilizers, as scale inhibitors, and scale deposit removers. Polyamino acids have been synthesized by thermal polycondensation reactions. Unfortunately, the known methods for the synthesis of polyamino acids are complex, expensive and provide low product yields.
A method for the polycondensation of aspartic acid in solid form is disclosed in E. Kokufuta et al., "Temperature Effect on the Molecular Weight and the Optical Purity of Anhydropolyaspartic Acid Prepared by Thermal Polycondensation", Bul. Chem. Soc. Japan, 61(5):1555-1556 (1978). The polycondensation reaction described therein requires heating solid L-aspartic acid to 160.degree. to 220.degree. C. for five hours. The product, anhydropolyaspartic acid (APAA), is then dialyzed for five to six days. Weight losses during the dialysis step range from 97% to 29%. The highest yield of APAA achieved by the Kokufuta et al. process is 50% by weight of APAA based on the starting weight of the aspartic acid at a reaction temperature of 220.degree..
Another method for the synthesis of polyaspartic acid is disclosed in Boehmke, U.S. Pat. No. 4,839,461. This process combines maleic acid or maleic anhydride and an ammonia solution in a molar ratio of 1:1-1.5. The mixture is then heated to 120.degree.-150.degree. C. and the resulting solution of ammonium salt and maleic acid is evaporated, leaving a crystal mash. The crystal mash is then melted, during which time the water of condensation and crystallization distill off. A porous mass of polyaspartic acid results. The entire process requires six to eight hours to complete.
The prior art methods for the synthesis of polyamino acids are time consuming and complex. It is an object of this invention to provide a relatively simple process for the manufacture of polyamino acids that results in a high percentage conversion of amino acid to polymer.